In order to disconnect or connect power transmission from a power source such as a vehicle internal combustion engine to a transmission, a clutch mechanism is provided between the power source and the transmission (refer to Patent Document 1). The clutch mechanism generally has a flywheel attached to an output shaft of the power source, and a clutch disc attached to an input shaft of the transmission. The clutch mechanism is coupled to a clutch pedal provided within a passenger compartment via a hydraulic piping or a cable. The clutch pedal is held at a non-depressed position by being normally urged by an urging force of a spring.
When the clutch pedal is not depressed, the clutch disc is pressed against the flywheel. At this time, friction force is generated between the clutch disc and the flywheel. Further, by means of the friction force, the clutch mechanism is connected so as to form an engaged state, and the power is transmitted from the power source to the transmission.
If the clutch pedal is depressed, the clutch disc is released from a state in which the clutch disc is pressed against the flywheel. At this time, the friction force generated between the clutch disc and the flywheel is reduced, and the clutch disc comes away from the flywheel. Accordingly, the clutch mechanism is disconnected so as to form a disengaged state, and the power transmission from the power source to the transmission is disconnected.
The clutch mechanism is activated through depressing operation of the clutch pedal by a driver. Accordingly, if depression of the clutch pedal is discontinued because a foot depressing the clutch pedal is removed from the clutch pedal or has slid in a lateral direction, the clutch pedal is rapidly returned to the non-depressed position, and the clutch mechanism rapidly shifts from the disengaged state to the engaged state.
Further, immediately after the clutch mechanism shifts from the disengaged state to the engaged state, the actual transmission torque transmitted to the input shaft of the transmission is changed as follows. The actual transmission torque temporarily overshoots a torque transmitted to the input shaft of the transmission at a steady state when the torque is fixed, that is, a steady state transmission torque. Thereafter, the actual transmission torque repeats decrease and increase at a predetermined period, the amplitude of the actual transmission torque becomes smaller and smaller, and the actual torque finally coincides with the steady state transmission torque.
At the transition as mentioned above, the greater the changing speed of the operating state of the clutch mechanism, that is, the greater the engaging speed of the clutch mechanism, the greater the amount by which the steady state transmission torque is overshot becomes, and the peak value of the actual transmission torque becomes large. Accordingly, if the clutch mechanism rapidly shifts from the disengaged state to the engaged state, the peak value of the actual transmission torque becomes large. Therefore, in order to secure the durability of a drive system including the transmission, it is necessary to set the strength of the drive system in correspondence to the peak value of the actual transmission torque. This reduces the flexibility of the design of the characteristic of the drive system.
If a system having a clutch mechanism of a small maximum engaging speed is employed, it is possible to reduce the peak value of the actual transmission torque. However, in this case, since the engaging speed is lowered in conditions other than the condition mentioned above, the response of the clutch mechanism is reduced.    Patent Document 1: Japanese Laid-Open Patent Publication No. 7-27217